1. Field of the Invention
The present invention relates to a vehicle braking system comprising: a master cylinder in which a master piston with its back turned to a boosted hydraulic pressure chamber is slidably housed in a casing; a backup piston which is slidably housed in the casing with its front face turned to the boosted hydraulic pressure chamber and with its backward limit fixed, to push the master piston directly from behind in response to operation of a brake operating member when the hydraulic pressure of the boosted hydraulic pressure chamber decreases; a hydraulic booster equipped with a control piston for achieving a balance between reaction force generated by hydraulic pressure of the boosted hydraulic pressure chamber and brake operating input from the brake operating member as well as with pressure control means contained in the backup piston to apply output hydraulic pressure of a hydraulic power source to the boosted hydraulic pressure chamber after regulating the output hydraulic pressure as the control piston moves axially; and a brake stroke simulator installed between the brake operating member and the control piston in order for a driver to feel operating strokes of the brake operating member; the master cylinder being connected to wheel brakes.
2. Description of the Related Art
Such a vehicle braking system is disclosed, for example, in Japanese Patent Publication No. 52-187.
In such a vehicle braking system, it is necessary to ensure that a master cylinder can generate brake hydraulic pressure through operation of a brake operating member, even if a hydraulic power source fails. In this case, it is desirable in terms of braking feel, to curb increases in a stroke and reaction of the brake operating member which would be invalidated by a brake stroke simulator. Therefore, it is conceivable to lock the brake stroke simulator by limiting the forward travel of the brake stroke simulator toward a control piston to a predetermined distance. However, when the hydraulic power source is operating normally, if spikes are inputted from the brake operating member with the hydraulic pressure of the boosted hydraulic pressure chamber at a low level, the brake stroke simulator may be locked erroneously, resulting in an insufficient stroke of the brake operating member.